1. Field of the Invention
The present invention relates to a wire cut method of shaping a workpiece by electric discharge. More particularly, it relates to an improved wire electrode offset treatment.
2. Description of the Prior Arts
It has been well-known to provide a wire cut method of shaping a workpiece by electric discharge to form a desired work configuration by relatively moving a workpiece to a wire electrode. This method is advantageous for shaping a workpiece in a complicated configuration in high accuracy.
FIG. 1 shows one embodiment of a conventional wire cut electric discharge machine wherein a wire electrode (12) is placed to face a workpiece (10) and a work liquid (14) is injected from a tank (16) through a pump (18) and a nozzle (20) into a work area. In order to carry out the relative movement of the workpiece (10) and the wire electrode (12) along the desired configuration, a table (22) on which the workpiece (10) is placed, is moved to two-dimensional directions along X-Y axis plane by an X-axis motor (24) and a Y-axis motor (26). A control signal is fed from a control device (28) to the motors (24), (26) whereby the relative movement of the workpiece (10) and the wire electrode (12) is controlled so as to correspond the desired configuration. The control device (28) can be an NC control device, a follow control device or a computer. The wire electrode (12) is taken out from a wire feed reel (30) and is fed to the workpiece by a current feeder (32) and an upper wire guide (34) and then, it is wound through a lower wire guide (36) on a wire winder and tension reel (38). An electric discharge energy is fed from a power source (40) for the work into a gap between the workpiece (10) and a wire electrode (12). The power source (40) for the work comprises a DC power source (42), a switching element (44) such as a transistor; a capacitor (46), a resistor (48) for charging and a switching element control circuit (50). The output voltage for the power source is fed from a current feeder (32).
FIG. 2 shows a pulse waveform output from the switching element (44) of the power source (40) for work shown in FIG. 1. In the waveform, a current peak value I.sub.p, a pulse length .tau..sub.p and a pulse quiescent length .tau..sub.r are shown.
In the conventional wire cut electric discharge machine, the electric discharge is formed in the gap between the workpiece (10) and the wire electrode (12) to work the workpiece (10) in a desired configuration by the electric discharge.
As shown in FIG. 3, a work groove (52) has a width given by the work condition. In order to give a precise work configuration, it is necessary to control the relative movement between the workpiece (10) and the wire electrode (12) depending upon the width of the work groove. Therefore, the wire electrode (12) should be controlled for the relative movement to the workpiece (10) at a position shifted for a desired offset from the desired configuration by the work. As it is clearly found in FIG. 3, the offset is given by a radius of the wire electrode (12) and a discharge gap G. A diameter of the wire electrode (12) is usually in a range of about 0.05 to 0.3 mm. This is precisely decided depending upon the kind of the wire electrode (12). The discharge gap G is remarkably varied depending upon the DC peak valve I.sub.p, the pulse length .tau..sub.p and the pulse quiescent length .tau..sub.r, shown in FIG. 2 and the electric work condition such as a gap voltage in the discharge gap; and the specific resistance of the work liquid, the kind and thickness of the workpiece (10) and the mechanical condition such as a work feeding speed. The discharge gap G is varied depending upon the fluctuation of these conditions in each operation. It is difficult to precisely decide the offset depending upon the variable discharge gap G. Therefore, it has not been attained to work a workpiece in a highly precise work configuration by the conventional machine.
In the conventional machine, a test work is carried out before each main work for shaping a workpiece in a desired work configuration by the electric discharge. The discharge gap G is measured on the test work to decide the offset. In accordance with the test work, however, it takes a long time for calculating the offset and it requires the complicated test work and it is not easy to provide a high accuracy because of individual difference for the measurement.